Problem: The scale on a map is 9cm : 10km. If the distance between two cities is 70km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 9cm on the map corresponds to an actual distance of 10km. An actual distance of 70km is the same as 7 $\cdot$ 10km. The distance between the two cities on the map is 7 $\cdot$ 9cm, or 63cm.